Modified fiber, methods, and systems

ABSTRACT

Methods of forming crosslinked cellulose include mixing a crosslinking agent with an aqueous mixture of cellulose fibers containing little to no excess water (e.g., solids content of 25-55%), drying the resulting mixture to 85-100% solids, then curing the dried mixture to crosslink the cellulose fibers. Systems include a mixing unit to form, from an aqueous mixture of unbonded cellulose fibers having a solids content of about 25-55% and a crosslinking agent, a substantially homogenous mixture of non-crosslinked, unbonded cellulose fibers and crosslinking agent; a drying unit to dry the substantially homogenous mixture to a consistency of 85-100%; and a curing unit and to cure the crosslinking agent to form dried and cured crosslinked cellulose fibers. Intrafiber crosslinked cellulose pulp fibers produced by such methods and/or systems have a chemical on pulp level of about 2-14% and an AFAQ capacity of at least 12.0 g/g.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 14/320,279,filed Jun. 30, 2014, the entire disclosure of which is herebyincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to methods of and systems for forming modifiedfiber, in particular intrafiber crosslinked cellulose fiber.

BACKGROUND

Traditionally, cellulose fibers from southern pine and other softwoodspecies are used in absorbent products in large part because themorphology of these fibers provides good absorbent performance. Comparedto hardwood fibers, southern pine and other softwood fibers tend to belonger (e.g., having a length weighted fiber length of about 2.5 mm) andmore coarse (e.g., having a coarseness greater than about 20 mg/100 m),and form low density pads with sufficient void volume to hold severaltimes their weight in liquid. Hardwood fibers, on the other hand, areknown for their performance in paper applications where shorter fiberlength (e.g., about 1 mm) and lower coarseness (e.g., less than about 20mg/100 m) provide a dense structure and smooth paper surface.

Crosslinked cellulose fibers are usually produced by applying acrosslinking agent to a dried sheet or roll of conventional softwoodpulp fibers, generally at a dilute concentration to ensure chemicalimpregnation of the sheet, followed by wet fiberization in a hammermillto generate treated, individualized cellulose fibers. These fibers arethen dried, such as in a flash drier, and cured, such as in an oven. Theresulting fibers exhibit intrafiber crosslinking in which the cellulosemolecules within a cellulose fiber are crosslinked. Intrafibercrosslinking generally imparts twist and curl to the cellulose fiber,and also imparts bulk to the fiber, properties that are advantageous insome absorbent products.

One drawback of this method is the high capital cost of the productionprocess, as well as high energy costs due to drying the fiber prior tocuring. Another drawback is that wet hammermilling can generate fiberand chemical buildup under usual mill conditions of heat and highairflow. Additionally, wet hammermilling produces undesirable featuressuch as knots, which are unfiberized fiber clumps or pieces of theoriginal pulp sheet. Generally, as production speeds increase, the levelof knots also increases as the hammermilling efficiency is reduced.

SUMMARY

Various embodiments of methods of forming crosslinked celluloseproducts, as well as crosslinked cellulose products formed therefrom,are disclosed herein. The products may include, for example, individualcrosslinked cellulose fibers, as well as mats, pads, sheets, webs, andthe like generally made from individual crosslinked cellulose fibers.

In one aspect, the present disclosure provides methods of formingcrosslinked cellulose products that include mixing a crosslinking agentwith an aqueous mixture of unbonded cellulose fibers having a solidscontent and containing little to no excess water. The crosslinking agentis added in an amount suitable to effect a desired level of crosslinkingin the cellulose fibers based on the solids content. In some methods,the mixing is performed at ambient conditions. In some methods, thecrosslinking agent is added in an amount no more than that required toeffect the desired level of crosslinking. The methods further includedrying the resulting mixture to 85-100% solids, then curing the driedmixture to crosslink the cellulose fibers. In some methods, the solidscontent of the aqueous mixture is about 25-55%. Some methods includeforming the aqueous mixture, such as by hydrapulping cellulose fibersfrom, for example, fibers provided in bale or roll form.

In one particular, non-limiting example of such a method, an aqueousmixture of unbonded cellulose fibers having a solids content of about40-50% is formed, followed by mixing a polyacrylic acid crosslinkingagent with the aqueous mixture in an amount to achieve a chemical onpulp level of about 2-14%, wherein said crosslinking agent is mixed atambient conditions. The resulting mixture is then dried and cured asabove.

In another aspect, the present disclosure provides embodiments of asystem for forming crosslinked cellulose products, which include a mixerconfigured to form, from an aqueous mixture of unbonded cellulose fibershaving a solids content of about 25-55% and a crosslinking agent, asubstantially homogenous mixture of non-crosslinked, unbonded cellulosefibers and crosslinking agent, at ambient conditions. The system furtherincludes, downstream of the mixer, a dryer configured to dry thesubstantially homogenous mixture to a consistency of 85-100% withoutcuring the crosslinking agent; and a curing unit coupled to the dryerthat is configured to cure the crosslinking agent, thereby forming driedand cured crosslinked cellulose fibers.

In another aspect, the present disclosure provides intrafibercrosslinked cellulose pulp fibers having a having a chemical on pulplevel of about 2-14% and an AFAQ capacity of at least 12.0 g/g. In someembodiments, the cellulose fibers are, or include, hardwood cellulosepulp fibers, such as eucalyptus cellulose pulp fibers.

The concepts, features, methods, and component configurations brieflydescribed above are clarified with reference to the accompanyingdrawings and detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an illustrative, non-limitingembodiment of a system suitable for producing crosslinked cellulosefibers in accordance with one aspect of the present disclosure.

FIG. 2 is a graph representing a relationship between AFAQ capacity andMUP capacity at 0.3 psi (2.07 kPa) load of several samples ofcrosslinked cellulose fibers prepared by methods in accordance with thepresent disclosure.

FIG. 3 is a graph representing a relationship between AFAQ capacity andCOP of crosslinking agent for representative samples prepared with a 20%polyacrylic acid crosslinking agent according to one aspect of thepresent disclosure, compared with samples prepared according to theconventional approach.

FIG. 4 is a graph representing a relationship between AFAQ capacity andCOP of crosslinking agent for representative samples prepared with avariety of crosslinking agents according to one aspect of the presentdisclosure.

FIG. 5 is a graph representing a relationship between AFAQ capacity andCOP of crosslinking agent for representative eucalyptus samples preparedaccording to one aspect of the present disclosure, compared withsouthern pine samples prepared according to the conventional approach.

DETAILED DESCRIPTION

According to one reference, U.S. Pat. No. 5,183,707 to Herron et al.,there are three basic crosslinking processes. The first may becharacterized as dry crosslinking, which is described, for example, inU.S. Pat. No. 3,224,926 to Bernardin. In a “dry crosslinking” process,individualized, crosslinked fibers are produced by impregnating swollenfibers in an aqueous solution with crosslinking agent, dewatering anddefiberizing the fibers by mechanical action, and drying the fibers atelevated temperature to effect crosslinking while the fibers are in asubstantially individual state. The fibers are inherently crosslinked inan unswollen, collapsed state as a result of being dehydrated prior tocrosslinking. These processes produce what are referred to as “drycrosslinked” fibers. Dry crosslinked fibers are generally highlystiffened by crosslink bonds, and absorbent structures made therefromexhibit relatively high wet and dry resilience. Dry crosslinked fibersare further characterized by low fluid retention values (FRV).

The second type, which is exemplified in U.S. Pat. No. 3,241,553 toSteiger, involves crosslinking the fibers in an aqueous solution thatcontains a crosslinking agent and a catalyst. Fibers produced in thismanner are referred to as “aqueous solution crosslinked” fibers. Due tothe swelling effect of water in cellulosic fibers, aqueous solutioncrosslinked fibers are crosslinked while in an uncollapsed, swollenstate. Relative to dry crosslinked fibers, aqueous solution crosslinkedfibers, for example as disclosed in the '553 patent, have greaterflexibility and less stiffness, and are characterized by higher fluidretention value (FRV). Absorbent structures made from aqueous solutioncrosslinked fibers exhibit lower wet and dry resilience than structuresmade from dry crosslinked fibers.

In the third type, which is exemplified in U.S. Pat. No. 4,035,147 toSangenis et al., individualized, crosslinked fibers are produced bycontacting dehydrated, nonswollen fibers with crosslinking agent andcatalyst in a substantially nonaqueous solution which contains aninsufficient amount of water to cause the fibers to swell. Crosslinkingoccurs while the fibers are in this substantially nonaqueous solution.This process produces fibers referred to herein as “nonaqueous solutioncrosslinked” fibers. Such fibers do not swell even upon extended contactwith solutions known to those skilled in the art as swelling reagents.Like dry crosslinked fibers, nonaqueous solution crosslinked fibers arehighly stiffened by crosslink bonds, and absorbent structures madetherefrom exhibit relatively high wet and dry resilience.

As explained in more detail herein, the present disclosure describes anadditional, more viable and flexible approach, as compared to the threedescribed by Herron.

In general, crosslinked cellulosic fibers can be prepared by applying acrosslinking agent(s) to cellulosic fibers in an amount sufficient toachieve intrafiber crosslinking under suitable conditions (e.g.,temperature, pressure, etc.). Several examples of polyacrylic acidcrosslinked cellulosic fibers and examples of methods for makingpolyacrylic acid crosslinked cellulosic fibers are described in U.S.Pat. Nos. 5,549,791, 5,998,511, and 6,306,251. A system and method thatmay be considered illustrative of the conventional approach to formingpolyacrylic acid crosslinked cellulosic fibers is disclosed, forexample, in U.S. Pat. Nos. 5,447,977 and 6,620,865. Accordingly,references to the “conventional approach” refer to the production ofcrosslinked cellulose fibers generally in accordance with that in theaforementioned patents, which follow the “dry crosslinking process” asdescribed by Herron. Briefly, the system in these patents includes aconveying device for transporting a mat or web of cellulose fibersthrough a fiber treatment zone, an applicator for applying acrosslinking agent to the fibers at the fiber treatment zone, afiberizer for separating the individual cellulose fibers comprising themat to form a fiber output comprised of substantially unbroken andessentially singulated cellulose fibers, a dryer coupled to thefiberizer for flash evaporating residual moisture, and a controlledtemperature zone for additional heating of fibers and an oven for curingthe crosslinking agent, to form dried and cured individualizedcrosslinked fibers.

Although current commercial processes for producing crosslinkedcellulose fiber products may use different reagents, reagent quantities,reaction and other process conditions, and so forth, than thosedisclosed in the aforementioned '977 and '865 patents, for the purposesof the present disclosure, references herein to the current commercialprocess generally refer to the conventional approach outlined in thesepatents.

Various aspects of the conventional approach are described in moredetail in the following paragraphs. The term “mat” refers to a nonwovensheet structure comprising cellulose fibers or other fibers that are notcovalently bound together, but are mechanically entangled and/or bondedby hydrogen bonds. The fibers include fibers obtained from wood pulp orother sources including cotton rag, hemp, grasses, cane, cornstalks,cornhusks, or other suitable sources of cellulose fibers that may belaid into a sheet. The mat of cellulose fibers is generally in sheetform, and may be one of a number of baled sheets of discrete size, ormay be a continuous roll.

Each mat of cellulose fibers is transported by a conveying device, whichcarries the mats through the fiber treatment zone, where a crosslinkingagent solution is applied to the mat. The crosslinking agent solution isapplied to one or both surfaces of the mat using methods includingspraying, rolling, dipping, etc. After the crosslinking agent solutionhas been applied, the solution may be uniformly distributed through themat, for example, by passing the mat through a pair of rollers.

The impregnated mat is then wet fiberized by feeding the mat through ahammermill. The hammermill disintegrates the mat into its componentindividual cellulose fibers, which are then air conveyed through adrying unit to remove the residual moisture.

The resulting treated pulp is then air conveyed through an additionalheating zone (e.g. a dryer) to bring the temperature of the pulp to thecure temperature. In one variant, the dryer includes a first drying zonefor receiving the fibers and removing residual moisture from the fibersvia a flash-drying method, and a second heating zone for curing thecrosslinking agent, to allow the chemical reaction (e.g.,esterification, in some embodiments), to be completed. Alternatively, inanother variant, the treated fibers are blown through a flash dryer toremove residual moisture, heated to a curing temperature, and thentransferred to an oven where the treated fibers are subsequently cured.Overall, the treated fibers are dried and then cured for a sufficienttime and at a sufficient temperature to achieve crosslinking.

As noted above, the conventional and historical approaches have somedisadvantages. For example, in the conventional (“dry crosslinking”)approach the crosslinking solution is generally very dilute (andcorrespondingly very low viscosity, generally lower than 5 cP) in orderto better assure complete impregnation of the chemical into the pulpsheet. The conventional method involves adding excess chemical, also tobetter assure complete impregnation, which presents additional chemicalhandling concerns. In addition, wet fiberization, such as by ahammermill, leads to fiber and chemical buildup under usual millconditions (sometimes referred to as contamination), which must beperiodically removed, requiring production downtime. In addition, wethammermilling tends to leave knots, with knot count generally increasingas production speeds increase, correspondingly decreasing hammermillingefficiency. Moreover, the conventional approach involves high energycosts due to wet hammermilling and water removal processes prior tocuring the fiber. A downside to aqueous solution crosslinking is that arecycle/reclaim loop for excess water and chemical is needed and must becontrolled.

Also, it has been found that the conventional approach is limited interms of the types of cellulose fibers suitable for effective use withthe dry crosslinking process, in which fiber mats are wetted with theaqueous crosslinking solution and then passed through rollers beforebeing fed to a hammermill and fiberized. Accordingly, fibers that do notform mats of sufficient integrity to withstand mechanical manipulationwhen impregnated with a liquid tend to be much more difficult, if notimpractical, to process efficiently on standard crosslinking equipment.As noted above, hardwood fibers are generally not used for absorbentproducts or in crosslinked cellulose fiber applications, because oftheir fiber morphology. In addition, however, some hardwood fibers, suchas eucalyptus, form mats that fall apart easily when wet, and thus arenot suitable fibers for use in the conventional approach.

As discussed in greater detail herein, systems and/or methods inaccordance with the present disclosure may circumvent the aforementioneddisadvantages, as well as providing an approach that can be used with acomparatively broader range of cellulose fibers. For example, mixing acrosslinking agent with an aqueous mixture of unbonded cellulose fibersthat contains little to no excess water can reduce dryer load and avoidthe contamination and knot content issues associated with wethammermilling. It also may reduce or eliminate the need for a chemicalrecycle loop. Further, it has been found that high solids contentaqueous fiber mixtures can be mixed with crosslinking agents of higherconcentrations than are used in the conventional approach, for examplein a high consistency mixer, and still achieve effective chemicaldistribution. This is unexpected, considering that materials having highsolids contents have comparatively higher viscosities (e.g., 10-50 cP orhigher), and it has traditionally been found to be difficult to achievesubstantially homogenous mixtures with fiber when combining highviscosity materials, especially within practical processing times. Inaddition, mechanical manipulation of an impregnated mat is not arequirement, which may further reduce capital cost as well as present anoption to crosslink cellulose fibers that have low wet tensile strengthor structural integrity, such as those from hardwood species such aseucalyptus, or cellulose fibers that are not available in sheet or matform. In addition, the methods of the present disclosure may be suitablefor cellulose fibers from plant species other than hardwood or softwoodtrees, as well as cellulose that has been treated (such as mercerizedfiber, and the like).

Mixtures of cellulose fibers and water, also referred to herein as“aqueous mixtures of cellulose fibers” (or, when it is clear that wateris present, simply “fiber”) exhibit different physical characteristicsat different concentration ranges. One way to characterize theconcentration ranges is by the manner in which water is retained bycellulose fibers. For example, cellulose fibers can bind a certainamount of water in pores within and on the surface of fibers, and inspaces between fibers. In general, water is more tightly bound insmaller pores (sometimes called micropores) than in larger pores(sometimes called macropores). Concentrations of such mixtures areconventionally expressed in terms of solids content, which refers, inthis context, to the weight of the cellulose divided by the weight ofthe mixture of cellulose and water.

In general, mixtures that have solids content of up to about 15-25%exhibit fluid characteristics. A mixture in this solids content range issometimes referred to as a slurry. Slurries can be drained of excesswater—that is, water that is not held between fibers by surface tensionforces or in fibrous pores—via gravitational forces and/or appliedvacuum.

At or above a solids content of about 25%, little to no excess waterremains, and the mixture is generally no longer flowable. Instead,mixtures at or above 25% solids content (and up to about 40-50%) oftentake the form of a moist, lumpy aggregate sometimes referred to ascrumb. Although no more water will drain from such a mixture, freewater—that is, water held between fibers and in large pores—may yet beremoved by mechanical pressure, such as by standard dewatering equipmentincluding screw presses, extruders, belt presses, roll presses, and soforth.

The cellulose fibers in the aforementioned slurry and crumb aqueousmixtures may be characterized as “unbonded”—that is, the fibers are notchemically bonded together, for example by covalent, hydrogen, or othertypes of chemical bonds, although there may be some mechanicalentanglement.

At a solids content of about 55%, the remaining water is bound withinthe fibers, generally within micropores, and must be removed byevaporation in order to achieve higher solids content levels. This isgenerally done by standard drying equipment such as ovens, float dryers,drum dryers, flash dryers, and so forth. Evaporation of bound water isgenerally accompanied by collapse of the fibers and formation ofhydrogen bonds, internal to, and/or between, fibers.

The threshold solids content levels that separate these threeconcentration ranges will vary to some extent among different types ofcellulose fiber, due to fiber species, the manner in which the wood waspulped to generate the fibers, whether and to what extent the pulp isrefined, and so forth.

Aqueous mixtures of cellulose fibers suitable for use in the presentdisclosure may be produced by any suitable method, such as by mixingcellulose, for example that is in roll or bale form, with water in ahydrapulper or a similar device, to a desired solids content. Thisprocess is sometimes referred to as slushing. Optionally, pulp in wetlap or other water-containing form (e.g., never-dried cellulose fibers)may be used, with water added or removed if necessary to achieve adesired solids content. In some methods, an aqueous mixture of a lowersolids content is made and then dewatered to a desired solids content,such as by means of an extruder. In some methods, the aqueous mixturemay be processed to remove or reduce fiber clumps, such as by feedingthe mixture, once a desired solids content has been achieved, throughone or more lump breakers, pin mills, or using other suitable means.

In methods in accordance with the present disclosure, the crosslinkingagent is added to the cellulose mixture at a concentration at whichthere is little to no excess water—that is, at a solids content range ofabout 25-55%. In a manner somewhat similar to that in the conventionalapproach to crosslinking discussed above, the presence of excess waterin the aqueous cellulose mixture when the crosslinking agent is addedrequires additional drying time, representing increased energy costs,and also may result in chemical buildup in the drying equipment,increasing the possibility of contamination and/or requiring downtimefor removal. In addition, adding crosslinking agent to a mixture thatcontains excess water (such as a slurry) may result in inadvertent lossof some of the crosslinking agent in solution as it drains from thecellulose fibers, which can be difficult to monitor and may reduceprocess efficiency.

Also, it is theorized that excess water allows the fibers to swell andallows some crosslinking agents to fully penetrate the cell wall. Thismay interfere with fiber stiffness, a desired quality in crosslinkedfibers. The concept of stiffening cellulose fiber is explained by theI-beam effect. Stiffer fibers are obtained when crosslinking is limitedto the surface of the fibers. Chemical that penetrates into the cellwall is less efficient at generating stiffness. Providing an excess ofchemical, due to low chemical solids (excess water), thus reduceschemical efficiency, as well as process efficiency by requiring achemical recovery or recycle loop, and so forth.

Accordingly, in the methods disclosed herein, the crosslinking agent isadded to the mixture of unbonded cellulose fibers and water at a solidscontent range of about 25-55%, which addresses many of the efficiencydrawbacks noted above. In accordance with the desire to increase processefficiency by reducing dryer load, a more preferred range is about35-55%. The capacity of current mixing equipment able to handlehigh-solids materials, such as high consistency mixers, tends tointroduce some practical limitations, and accordingly a most preferredrange is about 40-50%.

As used herein, the term “crosslinking agent” includes, but is notlimited to, any one of a number of crosslinking agents and crosslinkingcatalysts. The following is a representative list of useful crosslinkingagents and catalysts. Each of the patents noted below is expresslyincorporated herein by reference in its entirety.

Suitable urea-based crosslinking agents include substituted ureas suchas methylolated ureas, methylolated cyclic ureas, methylolated loweralkyl cyclic ureas, methylolated dihydroxy cyclic ureas, dihydroxycyclic ureas, and lower alkyl substituted cyclic ureas. Specificurea-based crosslinking agents include dimethyldihydroxy urea (DM DHU,1,3-dimethyl-4,5-dihydroxy-2-imidazolidinone),dimethyloldihydroxyethylene urea (DMDHEU,1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone), dimethylol urea(DMU, bis[N-hydroxymethyl]urea), dihydroxyethylene urea (DHEU,4,5-dihydroxy-2-imidazolidinone), dimethylolethylene urea (DMEU,1,3-dihydroxymethyl-2-imidazolidinone), and dimethyldihydroxyethyleneurea (DDI, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone).

Suitable crosslinking agents include dialdehydes such as C2-C8dialdehydes (e.g., glyoxal), C2-C8 dialdehyde acid analogs having atleast one aldehyde group, and oligomers of these aldehyde and dialdehydeacid analogs, as described in U.S. Pat. Nos. 4,822,453, 4,888,093,4,889,595, 4,889,596, 4,889,597, and 4,898,642. Other suitabledialdehyde crosslinking agents include those described in U.S. Pat. Nos.4,853,086, 4,900,324, and 5,843,061.

Other suitable crosslinking agents include aldehyde and urea-basedformaldehyde addition products. See, for example, U.S. Pat. Nos.3,224,926, 3,241,533, 3,932,209, 4,035,147, 3,756,913, 4,689,118,4,822,453, 3,440,135, 4,935,022, 3,819,470, and 3,658,613.

Suitable crosslinking agents include glyoxal adducts of ureas, forexample, U.S. Pat. Nos. 4,968,774, and glyoxal/cyclic urea adducts asdescribed in U.S. Pat. Nos. 4,285,690, 4,332,586, 4,396,391, 4,455,416,and 4,505,712.

Other suitable crosslinking agents include carboxylic acid crosslinkingagents such as polycarboxylic acids. Polycarboxylic acid crosslinkingagents (e.g., citric acid, propane tricarboxylic acid, and butanetetracarboxylic acid) and catalysts are described in U.S. Pat. Nos.3,526,048, 4,820,307, 4,936,865, 4,975,209, and 5,221,285. The use ofC2-C9 polycarboxylic acids that contain at least three carboxyl groups(e.g., citric acid and oxydisuccinic acid) as crosslinking agents isdescribed in U.S. Pat. Nos. 5,137,537, 5,183,707, 5,190,563, 5,562,740,and 5,873,979.

Polymeric polycarboxylic acids are also suitable crosslinking agents.Suitable polymeric polycarboxylic acid crosslinking agents are describedin U.S. Pat. Nos. 4,391,878, 4,420,368, 4,431,481, 5,049,235, 5,160,789,5,442,899, 5,698,074, 5,496,476, 5,496,477, 5,728,771, 5,705,475, andU.S. Pat. No. 5,981,739. Polyacrylic acid and related copolymers ascrosslinking agents are described in U.S. Pat. Nos. 5,447,977,5,549,791, 5,998,511, and 6,306,251. Polymaleic acid crosslinking agentsare also described in U.S. Pat. No. 5,998,511.

Specific suitable polycarboxylic acid crosslinking agents include citricacid, tartaric acid, malic acid, succinic acid, glutaric acid,citraconic acid, itaconic acid, tartrate monosuccinic acid, maleic acid,polyacrylic acid, polymethacrylic acid, polymaleic acid,polymethylvinylether-co-maleate copolymer,polymethylvinylether-co-itaconate copolymer, copolymers of acrylic acid,and copolymers of maleic acid.

Other suitable crosslinking agents are described in U.S. Pat. Nos.5,225,047, 5,366,591, 5,556,976, 5,536,369, 6,300,259, and 6,436,231.

Suitable catalysts can include acidic salts, such as ammonium chloride,ammonium sulfate, aluminum chloride, magnesium chloride, magnesiumnitrate, and alkali metal salts of phosphorous-containing acids. In oneembodiment, the crosslinking catalyst is sodium hypophosphite. Mixturesor blends of crosslinking agents and catalysts can also be used.

The crosslinking agent is added in an amount suitable to effect adesired level of crosslinking of the unbonded cellulose fibers based onthe solids content. The determination of a desired level of crosslinkingis often based on several considerations, such as a trade-off betweenincreased fiber stiffness due to crosslinking and diminished capillarypressure, as well as material and energy costs, handling concerns,production rates, and so forth. The amount of crosslinking agent may becharacterized as “chemical on pulp” (or “COP”), which refers to a masspercent. Some methods in accordance with this disclosure include addingthe crosslinking agent at a COP of about 2-14%, although other COPlevels and/or ranges are within the scope of this disclosure. Inaccordance with principles of process efficiency, in some methods, theamount of crosslinking agent is no more than is required to achieve thedesired level of crosslinking.

The concentration of the crosslinking agent is generally selected suchthat the addition of the agent to the aqueous mixture does not increasethe water content of the resulting mixture beyond the desired range. Forexample, a typical concentration range for polymeric crosslinking agentsis about 5-50%.

On the other hand, a premature decrease in the water content (that is,prior to drying) of the resulting mixture below the desired range mayalso have undesirable effects. With some crosslinking agents, waterremoval may result in the mixture becoming sticky and/or otherwisedifficult to handle, resulting in slower processing. One example of thismay be seen with polymeric crosslinking agents, in which a lack of watermay initiate polymerization, causing the solids content of the mixtureto increase and become sticky. Accordingly, in methods in accordancewith the present disclosure, the crosslinking agent is added to theaqueous mixture at ambient conditions, defined herein as a set ofconditions (e.g., temperature, pressure, air flow, time, etc.) underwhich water loss from the solution is minimized.

The crosslinking agent may be mixed with the aqueous mixture of unbondedcellulose fibers in any suitable manner, such as in a high consistencymixer, an extruder (or a region of an extruder, such as a section of anextruder downstream of a dewatering section), a refiner, and so forth.One advantage to the use of a high consistency mixer, in someembodiments, is that a high consistency mixer not only allows directinjection of the crosslinking chemistry into the mixture at solidscontents of up to about 50%, but the mixer also may also fluff the fiberto prepare it for drying. Once mixed, the methods of the presentdisclosure include drying the mixture to about 85-100% solids, such aswith standard drying apparatus (e.g., flash dryers, jet dryers, ringdryers, and so forth, or combinations thereof).

Curing refers to the initiation and ensuing chemical reaction thatcreates chemical bonds between the crosslinking agent and the cellulose.Crosslinking occurs by different chemical reactions, depending on thecrosslinking agent. For example, polyacrylic and polycarboxylic acidcrosslinking agents typically establish chemical crosslinks by means ofan esterification reaction when reacted with cellulose. The presentdisclosure encompasses methods that proceed not only by esterificationcrosslinking reactions, but also by other crosslinking reactions, suchas etherification and so forth, as well as the reaction conditionssuitable for such reactions. Methods in accordance with the presentdisclosure proceed by curing the dried mixture under conditionseffective to crosslink the unbonded cellulose fiber in the mixture.Curing may be accomplished by any suitable manner, such as those used inthe conventional approach, etc.

With the illustrative methods discussed above in mind, including thevarious steps, concepts, and variants therein, FIG. 1 can be seen to bea schematic representation of an illustrative, non-limiting embodimentof a system, generally indicated at 10, that is suitable for producingcrosslinked cellulosic compositions in accordance with aspects of thepresent disclosure.

System 10 is shown in FIG. 1 to include a series of boxes connected byarrows. As will be described, the boxes represent different functionalregions, or units, of system 10. The boxes, as well as the term “unit,”are used for convenience, as each functional unit may be a singlecomponent (such as a machine, piece of equipment, apparatus, and soforth), or part of a larger component that also incorporates one or moreother functional units, or may represent multiple components thatcooperate to perform the function(s) of the unit, and so forth. Variousfunctional units and components of system 10 may be co-located, such aswithin a single facility (such as a mill), or located remotely from eachother. The system 10 may be any suitable scale, from lab scale toindustrial/commercial. The arrows generally represent the direction ofthe material or product produced or processed by the various functionalunits, and, accordingly, may also represent any suitable means ofconveying the material from one unit to another (such as conduits,conveyors, etc.), and/or other pieces of processing or handlingequipment.

System 10 is shown to include a mixing unit 20 that is configured toform, from an aqueous mixture of unbonded cellulose fibers having asolids content of about 25-55% and a crosslinking agent, a substantiallyhomogenous mixture of non-crosslinked, unbonded cellulose fibers andcrosslinking agent, at ambient conditions. As noted above, the mixingunit 20 may thus include, for example, a high consistency mixer orrefiner to which an aqueous fiber mixture and crosslinking agent areadded, as well as any necessary metering and/or delivery equipment forthe mixture components. Accordingly, the fiber 22 and water 24 may beprovided as a mixture, such as an aqueous mixture having the desiredsolids content, for example in embodiments in which the aqueous mixtureis formed upstream of the mixing unit 20, and then mixed in mixing unit20 with a crosslinking agent 26. As noted above, equipment (not shown)such as a hydrapulper, extruder, or other suitable equipment may producethe mixture. Prior to introduction to the mixing unit 20, such a mixturemay be passed, for example, through one or more pieces of dewatering,processing, and/or handling equipment (not shown), such as one or morepin mills, screw presses, refiners, lump breakers, surge bins orhoppers, conveyors, and so forth.

Optionally, in some embodiments, the mixing unit 20 may incorporate suchequipment, and be configured to accept the fiber 22 and water 24 asseparate materials, such as to produce an aqueous mixture that is thenmixed with crosslinking agent 26. In such embodiments, the mixing unitmay be characterized as including a first zone (not separately shown)that is configured to form the aqueous mixture as described above, and asecond zone (not separately shown) that is configured to receive boththe aqueous mixture and the crosslinking agent and form thesubstantially homogenous mixture. As an example of such an embodiment,the first and second zones may be two subsequent regions of an extruder.In embodiments in which the aqueous mixture is produced, for example, atlow solids content and then dewatered to the desired solids content formixing with the crosslinking chemical, the mixing unit 20 may include awater recycle/reclaim loop, such as from a dewatering device to ahydrapulper.

The mixing unit is configured to mix the aqueous fiber mixture with thecrosslinking agent, which as noted above may include one or morecrosslinking chemicals and/or catalysts, as desired, under ambientconditions, that is, process conditions such as temperature, pressure,air flow, time, etc., under which water loss from the solution isminimized. The term “substantially homogenous,” when used to describethe mixture of cellulose fibers, water, and crosslinking agent,indicates that the crosslinking agent is sufficiently well distributedamong the fiber so as to form consistent and uniform crosslinksthroughout each fiber when dried and cured. As noted above, the mixingunit, such as in embodiments in which the mixing unit includes a highconsistency mixer, may also fluff the fiber (that is, impart an increasein bulk density) in the mixture. Optionally, the mixing unit may includeother equipment to fluff the mixture prior to drying.

Downstream of mixing unit 20 is a drying unit 30 configured to receivethe mixture from the mixing unit and dry the mixture to 85-100% solids.Accordingly, drying unit 30 may include one or more drying devices, suchas one or more ovens, float dryers, drum dryers, flash dryers, jetdryers, and so forth. In some embodiments, the drying unit 30 may alsobring the fibers up to or near to curing temperature.

Finally, the dried fibers are received by a curing unit 40 configured tocure the crosslinking agent, thereby forming dried and crosslinkedcellulose fibers. The curing unit thus may incorporate additional dryingdevices, ovens, and so forth. In some embodiments, the drying unitand/or curing unit may incorporate a holding area, such as to allow thefibers to equilibrate at a set temperature and/or time, or suchequilibration may occur as the fibers are conveyed from one functionalunit to the next. Some embodiments may include a recycle/reclaim loopfor air/heat from curing equipment to drying equipment.

Once formed, the crosslinked fibers exit the curing unit 40 and may besubjected to various post treatment processes, indicated generally at50, such as to prepare the fibers for shipment or storage, for exampleby being baled according to standard methods, which may includeremoisturizing followed by baling, and so forth.

Absorbent properties of crosslinked cellulosic compositions (such as wetbulk, wick time, wick rate, absorbent capacity, and so forth), may bedetermined using the Automatic Fiber Absorption Quality (AFAQ) Analyzer(Weyerhaeuser Co., Federal Way, Wash.). A standard testing procedure isdescribed in the following paragraphs.

A 4-gram sample (conditioned at 50% RH and 73° F. (23° C.) for at least4 hours) of the pulp composition is placed through a pinmill to open thepulp, and then airlaid into a tube. The tube is placed in the AFAQAnalyzer. A plunger then descends on the airlaid fluff pad at a pressureof 0.6 kPa. The pad height is measured, and the pad bulk (or volumeoccupied by the sample) is determined from the pad height. The weight isincreased to achieve a pressure of 2.5 kPa and the bulk is recalculated.The result is two bulk measurements on the dry fluff pulp at twodifferent pressures.

While under the plunger at the higher pressure, water is introduced intothe bottom of the tube (to the bottom of the pad), and the time requiredfor water to wick upward through the pad to reach the plunger ismeasured. From this, wick time and wick rate may be determined. The bulkof the wet pad at 2.5 kPa may also be calculated. The plunger is thenwithdrawn from the tube, and the wet pad is allowed to expand for 60seconds. In general, the more resilient the sample, the more it willexpand to reach its wet rest state. Once expanded, this resiliency ismeasured by reapplying the plunger to the wet pad at 0.6 kPa anddetermining the bulk. The final bulk of the wet pad at 0.6 kPa isconsidered to be the “wet bulk at 0.6 kPa” (in cm³/g, indicating volumeoccupied by the wet pad, per weight of the wet pad, under the 0.6 kPaplunger load) of the pulp composition. Absorbent capacity (or “AFAQcapacity”) may be calculated by weighing the wet pad after water isdrained from the equipment, and is reported as grams water per gram drypulp.

Maximum uptake (“MUP”) of a fiber sample is also a capacity type value,but as measured under a different load. Capillary pressure measurementsof a sample porous material are made on a TRI/Autoporosimeter(TRI/Princeton Inc. of Princeton, N.J.) to determine pore volumes andpore size distributions (see, e.g., EP2407133A1; see also The Journal ofColloid and Interface Science, Vol. 162, Issue 1 (January 1994), pp163-170; the disclosures of both are incorporated herein by reference).As used in this application, determining the capillary pressurehysteresis curve of a material as function of saturation involvesrecording the increment of liquid that enters a porous material as thesurrounding air pressure changes. A sample in the test chamber isexposed to precisely controlled changes in air pressure which atequilibrium (no more liquid uptake/release) correspond to the capillarypressure. The equipment operates by changing the test chamber airpressure in user-specified increments, either by decreasing pressure(increasing pore size) to absorb liquid, or increasing pressure(decreasing pore size) to drain liquid. The liquid volume absorbed(drained) is measured with a balance at each pressure increment.

A standard testing procedure is performed at 23° C.±2 ° C. (73° F.) anda relative humidity of 50%±5%. The test is run using a 0.9% salinesolution. The surface tension (mN/m), contact angle (°), and density(g/cc) is determined by any method known in the art and provided to intothe instrument's software (in this case the values used are 72, 0, and1, respectively). The balance leveled at 156.7 g and equilibration rateset to 90 mg/min. The pore radius protocol (corresponding to capillarypressure steps) to scan capillary pressures, according to equation R=2ycos θ/Δp, is assigned, where: R is the pore radius, y is the surfacetension, θ is the contact angle, and Δp is the capillary pressure. Forexample, a set of pore radius (R) steps for first absorption (pressuredecreasing) are 25, 74, 98, 108, 120, 136, 156, 184, 245, 368, 735,1470, 2940; and for desorption (pressure increasing) are 1470, 735, 490,368, 147, 82, 65, 54, 47, 42, 25. A 0.5 g sample is cut into a 52 mmdiameter circular specimen, then conditioned at 23° C.±2° C. (73° F.)and a relative humidity 50%±5% for minimum four (4) hours beforetesting. The weight is measured, to ±0.0001, and the specimen is placedat the center of the membrane (MF™ cellulose nitrate membrane filtertype 8.0 micron SCWP available from Merck Millipore Ltd., Cork,Ireland). The desired load (0.2 psi or 1.38 kPa) is added onto thesample and the chamber is closed. After the instrument has applied theappropriate air pressure to the cell, the liquid valve to allow freemovement of liquid to the balance is opened and the test under theradius protocol begins. The instrument proceeds through oneabsorption/desorption cycle. A blank (without sample specimen) is run inlike fashion.

The mass uptake from a blank run is directly subtracted from the uptakeof the sample at each target pore radius (pressure). The maximum uptakeis the maximum value of liquid absorbed by the sample that correspondsto the lowest pressure. Saturation at each capillary pressure step isautomatically calculated from liquid uptake as follows: S=m^(l)/m^(l)_(max) where: S=saturation, m^(l)=liquid uptake at the pressure step(mL), and m^(l) _(max)=maximum liquid uptake (mL). Pressure is reportedin cm of water and saturation in %. Data from the first absorption curveand the desorption curve are used. The liquid absorbed by the sample at100% saturation is the maximum uptake. The maximum liquid uptake in mLprovided by the instrument is divided by the liquid density to providethe liquid weight in grams. The maximum uptake in grams is divided bythe dry sample weight in grams to obtain the reported value in g/g.

The aforementioned example embodiments are illustrative of any number ofsuitable application methods, as well as combinations thereof, all ofwhich are understood to be encompassed by the present disclosure.

The following examples summarize representative, non-limitingembodiments and methods of forming crosslinked cellulose products inaccordance with the methods and concepts discussed above, and areillustrative in nature. The reagent amounts, times, conditions, andother process conditions may be varied from those disclosed in thespecific representative procedures disclosed in the following exampleswithout departing from the scope of the present disclosure.

EXAMPLE 1 Lab Scale with PAA and Variable Fiber Solids Content

Southern pine fiber (CF416, Weyerhaeuser NR Company) was slushed in alaboratory pulper in 1000 g (OD) batches and then dewatered in alaboratory centrifuge. The dewatered fiber was broken down into smallerfiber bundles using a laboratory pin mill. The solids content of thefiber was measured, and then the desired amount of fiber for the testwas fed via conveyor into a hopper. A screw at the bottom of the hopperfed the fiber into a laboratory Sprout refiner fitted with traditionalrefining plates (C2976) in a vertical configuration, with the gap set tominimize any fiber cutting (generally 0.050″-0.300″). Crosslinkingchemical (polyacrylic acid (“PAA”) polymer) at 20% solids content wasapplied via a chemical port located at the end of the screw immediatelybefore the fiber enters the refiner. Fiber was delivered at a fixedrate. The chemical pump speed was changed to achieve the requiredchemical addition level. The treated fiber exited the refiner into aplastic beaker. The fiber was dried in a Fluid Energy 4″ ThermaJet™ jetdryer with a target inlet temperature of 356° F. (180° C.). Dried fiberwas equilibrated at room temperature before curing at 370° F. (187.8°C.) for 5 minutes.

Samples were compared to a commercial control (CMC530, available fromWeyerhaeuser NR Company) prepared under the same chemical loading andcuring conditions, but according to the conventional method.Representative samples and their corresponding AFAQ capacity results atconstant COP are shown in Table 1 (Sample UC represents the untreatedcontrol, and Sample CC-PAA represents the commercial control using PAA).Table 1 indicates not only that effective crosslinking was achieved athigh solids, but also that the AFAQ capacity of samples preparedaccording to the high solids methods of the present disclosure isunexpectedly greater as compared to samples prepared according to theconventional method.

TABLE 1 Incoming Fiber Sample ID Solids, % AFAQ capacity (g/g)Crosslinked? Sample UC n/a 12.6 No Sample CC-PAA n/a 16.7 Yes Sample 1A35.0 18.7 Yes Sample 1B 39.1 18.9 Yes Sample 1C 41.6 18.9 Yes Sample 1D43.8 18.4 Yes Sample 1E 43.9 18.6 Yes

EXAMPLE 2 Lab Scale with Citric Acid

The process of Example 1 was followed using a 18% polycarboxylic acid(citric acid) solution as the crosslinking agent. Average solids contentof the fiber was 43.3%. Curing conditions were 340° F. (171.1° C.) for 5minutes. AFAQ capacity results for a representative sample are comparedwith a commercial control (CMC520, available from Weyerhaeuser NRCompany) using the same citric acid crosslinking agent and with anuntreated control in Table 2 (Sample CC-citric represents the commercialcontrol using citric acid crosslinker). Again, the AFAQ capacity ofSample 2, prepared according to the high solids methods of the presentdisclosure, is unexpectedly greater as compared to the commercialcontrol.

TABLE 2 Sample ID AFAQ capacity (g/g) Crosslinked? Sample UC 12.6 NoSample CC-citric 14.7 Yes Sample 2 15.7 Yes

EXAMPLE 3 Lab Scale with Variable PAA Solids Content

The process of Example 1 was conducted over an incoming solids contentrange of the polyacrylic acid crosslinking agent of 16.8-40% whilekeeping the incoming solids content of the fiber constant. For eachsample, several levels of crosslinking agent were applied. To assessperformance, the relationship between two capacity methods (maximumuptake, or “MUP,” and AFAQ capacity) was compared. For all samples, thesame relationship was observed to apply regardless of the beginningsolution strength or crosslinking agent.

FIG. 2 is a graph generally showing the correlation between AFAQcapacity (which increases from left to right on the x-axis) and MUPcapacity at 0.3 psi (2.07 kPa) load (which increases from bottom to topon the y-axis) of several samples prepared as described in Example 3(i.e., with PAA crosslinking agent, over a range of COP of about 2-14%),and also includes samples prepared as described in Example 2 (i.e., with18% citric acid crosslinking agent, similar COP range), as well as labcontrols.

EXAMPLE 4 Lab Scale with Alternative Process Configuration

Southern pine fiber (CF416, Weyerhaeuser NR Company) was slushed in alaboratory pulper in 1000 g (OD) batches, then dewatered in a laboratorycentrifuge. The dewatered fiber was broken down into smaller fiberbundles using a laboratory pin mill. The solids content of the fiber wasmeasured to be 43.8%. This fiber was fed via conveyor into a hopper. Ascrew at the bottom of the hopper fed the fiber into a laboratory Sproutrefiner fitted with “devil tooth” mixing plates (C2975A), with the gapwas set to minimize any fiber cutting. PAA crosslinking chemical at 20%solids content was applied via a chemical port located at the end of thescrew immediately before the fiber enters the refiner. Chemical wasdelivered at a fixed rate. The conveyor speed was changed to achieve thetarget fiber feed rate to provide the required chemical dosage. Thetreated fiber exited the refiner into a plastic bucket. The fiber wasdried in a Fluid Energy Processing & Equipment Company 4″ ThermaJet™dryer with a target inlet temperature of 356° F. (180° C.). Dried fiberwas equilibrated at room temperature before curing at 370° F. (187.8°C.) for 5 minutes.

The product was compared to a commercial control (CMC530) prepared underthe same chemical loading and curing conditions. The results are shownin Table 3, in which Sample 4 represents the product and Sample 1D datais also included for comparison.

TABLE 3 Incoming Fiber Sample ID Solids, % AFAQ capacity (g/g)Crosslinked? Sample UC n/a 12.6 No Sample CC-PAA n/a 16.7 Yes Sample 443.8 16.8 Yes Sample 1D 43.8 18.4 Yes

EXAMPLE 5 Pilot Scale

A pilot scale trial using a process in accordance with that of Example 1was conducted. Southern pine fiber (CF416, Weyerhaeuser NR Company) wasslushed in a pilot pulper (Black Clawson 300 gallon capacity) and thenfed to a commercial screw press (Press Technology and Manufacturing Inc,Model 08L200) for dewatering. Fiber chunks from the screw press werebroken apart by a rotary pin mill. The solids content of the fiber wasmeasured to be 41.6%. The fiber was placed into a volumetric feeder witha 6″ screw. Metered fiber was dropped onto a conveyer that deposited thefiber into the inlet of an high consistency (“HC”) mixer (manufacturedby Andritz) using C2975A mixer plates in a horizontal configuration withthe gap was set to minimize any fiber cutting. Crosslinking chemical(22.3% PAA solution) was pumped into the inlet of the mixer via aninjection port. The chemical feed rate was adjusted to provide therequired chemical addition level. The treated fiber exited thecommercial mixer into a drum, and then dried in a Fluid EnergyProcessing & Equipment Company 4″ ThermaJet™ dryer. Dried fiber wasequilibrated at room temperature before curing at 380° F. (193.3° C.)for 8 minutes. Performance of the pilot scale trial matched the labresults at the same target chemical loading and curing conditions, asshown in Table 4.

TABLE 4 Incoming Fiber Sample ID Solids (%) AFAQ capacity (g/g)Crosslinked? Sample 5 (pilot) 41.6 17.7 Yes Sample 5 (lab) 41.6 17.4 Yes

EXAMPLE 6 Lab Scale with Variable COP

The process of Example 1 was conducted over a range of crosslinkingchemical addition levels, using 20% PAA. As shown in FIG. 3, AFAQcapacity for such fibers over a range of about 2-14% was found to beimproved over crosslinked cellulose fibers produced according to thecommercial method under similar conditions (of reagent amounts, times,and so forth). Samples prepared according to Example 1 are indicated as“new process” data points, whereas the commercial method preparedsamples are indicated as “current process” data points.

FIG. 4 shows the general trend of AFAQ capacity (which increases frombottom to top on the y-axis) to increase as COP increases over the rangetested, with both PAA crosslinking agents (over a range of solidscontent of the crosslinking agent) and citric acid crosslinking agent.

EXAMPLE 7A Alternative Pulps

Never-dried Douglas fir wet lap, obtained from Weyerhaeuser, wasslushed, dewatered, processed, mixed with PAA at 20%, and dried andcured in accordance with the process of Example 6. Because theconventional process uses once-dried pulp sheets, a sample following theconventional process was not prepared with wet lap (which is never-driedpulp). Results (representative sample shown in Table 5) showed favorableAFAQ capacity of crosslinked fibers formed from wet lap.

TABLE 5 Incoming Fiber Sample ID Solids (%) AFAQ capacity (g/g)Crosslinked? Sample 7-FIR 37.7 17.0 Yes

EXAMPLE 7B Alternative Pulps

Eucalyptus pulp (Bleached Eucalyptus Kraft Pulp available from FibriaVeracel mill, Brazil) was processed in accordance with the process ofExample 4. Representative samples (Sample 7-EUC represents eucalyptus,and Sample UC-EUC represents untreated eucalyptus) are shown in Table 6.For comparison, Table 6 also includes data for a commercial control(CMC530, indicated as Sample CC-PAA), and a lab control prepared fromCF416 and crosslinked according to the same method at the same COP asSample 7-EUC (indicated as Sample LC-PAA).

TABLE 6 Incoming Fiber Sample ID Solids (%) AFAQ capacity (g/g)Crosslinked? Sample CC-PAA n/a 16.7 Yes Sample LC-PAA 41.8 16.8 YesSample UC-EUC n/a 12.0 Yes Sample 7-EUC 41.2 17.4 Yes

Table 6 indicates not only that effective crosslinking was achieved witheucalyptus pulp, but also that the AFAQ capacity thereof exceeds that ofconventionally produced southern pine crosslink.

Several samples were prepared, varying the COP levels for the eucalyptusfibers. FIG. 5 is a graph showing that the AFAQ capacity of eucalyptusfibers crosslinked according to methods of the present disclosure(indicated as “Eucalyptus” in FIG. 5) were superior to that of southernpine kraft pulp produced according to the conventional crosslink method(indicated as “Southern Pine” in FIG. 5) over a sample COP range.

Although the present invention has been shown and described withreference to the foregoing operational principles and illustratedexamples and embodiments, it will be apparent to those skilled in theart that various changes in form and detail may be made withoutdeparting from the spirit and scope of the invention. The presentinvention is intended to embrace all such alternatives, modificationsand variances that fall within the scope of the appended claims.

What is claimed is:
 1. Intrafiber crosslinked cellulose pulp fibers having a chemical on pulp level of 2-14% and an automatic fiber absorption quality (“AFAQ”) capacity of at least 12.0 g/g.
 2. The intrafiber crosslinked cellulose pulp fibers of claim 1, wherein the fibers include hardwood cellulose pulp fibers.
 3. The intrafiber crosslinked cellulose pulp fibers of claim 2, wherein the fibers are hardwood cellulose pulp fibers.
 4. The intrafiber crosslinked cellulose pulp fibers of claim 2, wherein the hardwood cellulose pulp fibers are eucalyptus pulp fibers.
 5. The intrafiber crosslinked cellulose pulp fibers of claim 1, wherein the fibers are intrafiber crosslinked with a polyacrylic acid and/or polycarboxylic acid crosslinking agent.
 6. The intrafiber crosslinked cellulose pulp fibers of claim 1, having an AFAQ capacity of at least 16.0 g/g. 